Inorganic thin films such as silicon oxide (SiO2) and SiN are used in various semiconductor devices, solar batteries or the like as an interlayer insulating film, a passivation film, an anti-reflection film, or the like.
For example, in some solar batteries, an anti-reflection film made of a SiN film is formed on the light-receiving side and a passivation film is formed on the side opposite to the light-receiving side in order to improve conversion efficiency.
In production of a crystallized silicon solar battery cell that is a common solar battery, a phosphorus-diffused layer as the n+ layer is formed on the surface layer of p-type silicon wafer, and between the n+ layer and the p layer as an underneath layer, a pn junction is formed. Subsequently, after forming an anti-reflection film on the n+ layer, an electrode is formed on each of the light-receiving side and the back side, and, if necessary, a passivation film is formed.
In production of a solar cell, there are many cases where an electrode is connected to an n+ layer after forming an anti-reflection film or a passivation film. Therefore, it is required to form openings in these inorganic thin films.
As one of methods for forming an opening in an inorganic thin film, an etching method can be given. A method using a photoresist is a common etching method. However, this method is not efficient since it requires steps of forming a resist film, exposure, development, etching and removing of the resist, and uses a large number of materials.
Further, a method can be mentioned in which an opening is formed by means of a laser. In this method, however, control of processing positions is complicated, leading to a prolonged processing time, whereby productivity is insufficient. In addition, the underlying n+ layer, the wafer or the like may be damaged by the laser.
In addition, conventionally, as the most common production method, a conductive paste that contains a metal that is used as an electrode and a compound that constitutes glass, such as silicon oxide, is applied, and “firethrough” is caused to occur by heating, whereby an opening is formed in an inorganic thin film, and at the same time, the electrode is connected to the n+ layer. In this method, however, since a high-temperature treatment at a temperature of 250° C. or higher is required, the n+ layer or the wafer may be damaged, leading to lowering in power generation efficiency.
Meanwhile, a method can be thought that an inorganic thin film is patterned from the beginning. This method is not efficient since the steps are complicated, and pattern formation is not sufficient in respect of accuracy.
On the other hand, a method has been proposed in which an etching paste is printed to form a pattern or the like on an inorganic thin film, followed by heating, whereby an opening is formed beneath the etching paste.
As components for removing an inorganic thin film (etching components), for example, Patent Document 1 discloses an etching medium that contains phosphoric acid or phosphate. However, since the optimum etching temperature for this component is as high as 250° C. or higher, the n+ layer, wafer or the like may be damaged.
A method is disclosed (see Patent Document 2) in which an etching medium that contains at least one fluorine compound selected from a fluoride of ammonium, an alkali metal and antimony; an acid fluoride of ammonium, an alkali metal and calcium; alkylated ammonium and potassium tetrafluoroborate, and optionally, a prescribed inorganic mineral acid and a prescribed organic acid. This method, however, has a problem that an etching medium as a mixture of a fluorine compound and an acid has significant toxicity.